With the recent spread of mobile electronic devices such as mobile video cameras and mobile PCs, demand for secondary batteries as a portable power source is rapidly increasing. In addition, with respect to such secondary batteries, there is a great need for achieving downsizing, weight reduction, and enhanced energy density.
Conventionally, aqueous batteries such as lead batteries and nickel-cadmium batteries have been mainly used as secondary batteries which can be repeatedly charged and discharged. Though these aqueous solution-based batteries have excellent charge/discharge characteristics, they do not sufficiently satisfy the properties as a portable power source for a mobile electronic device in terms of the battery weight and energy density.
Lithium secondary batteries including a negative electrode made of lithium or a lithium alloy have been thus intensively studied and developed as secondary batteries. The lithium secondary batteries have excellent properties such as high energy density, less self-discharge, and light weight.
An electrode of a lithium secondary battery is commonly prepared as follows. An active material and a binder are kneaded together with a solvent so that the active material is dispersed, thereby preparing a slurry. The slurry is applied to a current collector by the doctor blade method or the like, and dried to form a thin film as an electrode.
At the present, a fluororesin typified by polyvinylidene fluoride (PVDF) is most widely used as a binder for an electrode of a lithium secondary battery.
In the case of using a fluororesin as a binder, though a flexible thin film can be prepared, binding between a current collector and an active material is poor. Due to this, the active material may partially or entirely be detached or fall off during a process of producing a battery. Moreover, upon charge and discharge of the battery, insertion and release of lithium ions are repeated in the active material. Due to this phenomenon, the active material may be detached or fall off from the current collector.
To solve such a problem, a binder other than PVDF has been tried to be used. In the case of using a conventional resin, however, decomposition or deterioration of the resin may occur upon application of a voltage to an electrode. When the resin is decomposed or deteriorated, the charge/discharge capacity may be lowered and the electrode may be detached.
To overcome such a situation, Patent Literature 1 discloses a binder for a non-aqueous secondary battery which includes a copolymer prepared from an acidic functional group-containing monomer and an amide group-containing monomer.
In the case of using such a binder, however, the dispersibility of an active material is lowered and the viscosity of the resulting composition for an electrode increases. In such a case, paste filtration takes time to prolong the preparation time and coating unevenness is likely to be developed upon application of the composition. In addition, since the active material density in the electrode is lowered, the capacity of the resulting battery is insufficient.
Moreover, the use of such a resin lowers the flexibility of the resulting electrode to cause cracking or peeling thereof from the current collector, leading to poor battery durability.
Patent Literature 2 discloses a composition of a binder for a secondary battery positive electrode containing an aromatic vinyl unit, a nitrile group unit, a hydrophilic group unit, and a linear alkylene unit each in a predetermined amount.
Even in the case of using such a composition, the dispersibility of an active material is lowered and the viscosity of the resulting composition for an electrode increases. Accordingly, paste filtration takes time to prolong the preparation time and coating unevenness is likely to be developed upon application of the composition. In particular, an increase in the viscosity is significant when the time has passed after preparation of the composition for an electrode.
In addition, since the active material density in the electrode is lowered, the capacity of the resulting battery is insufficient.